Do not eagerly discover CWD during parser construction

[baronfel]

As reported here, some string arguments set their default value to the current directory. The way this is currently configured looks up and sets this value at parser-construction time as opposed to command-invocation time, which has a few side effects that we should change:

• this work is done for every parse, instead of deferring to just parses that invoke the Command that uses the Argument configured in this way
• this work can potentially fail, giving unexpected errors to the user - in the linked thread @khalidabuhakmeh was running a workload command, but the stack trace came from the add command

[KalleOlaviNiemitalo]

Does System.CommandLine provide a way to avoid even constructing the Argument instances of subcommands other than the subcommand chosen by the user?